There are many known types of welding-type power supplies that provide welding type power. Welding type power, as used herein, refers to power suitable for electric arc welding, plasma arc cutting or induction heating. Welding-type power supply, as used herein, refers to a power supply that can provide welding type power. Welding type systems are used to perform a variety of processes and used in a variety of settings. Welding-type system, as used herein, is a system that can provide welding type power, and can include control and power circuitry, wire feeders, and ancillary equipment.
Some welding type systems include an input circuit and/or a preregulator which provide a dc bus, followed by an inverter based output circuit. The preregulator conditions the input power, and provides a known dc bus. The inverter based output circuit receives the bus and provides the welding type power as an output. One successful design includes a boost circuit as part of the preregulator, and the output circuit includes an inverter, transformer, rectifier and output inductor. This type of welding type power supply is described in U.S. Pat. No. 6,987,242 (Geissler). Other welding type power supplies that have inverter based output circuits include U.S. Pat. No. 6,115,273 (Geissler) and Patent Publication 20090230941 (Vogel), all three of which are owned by the owner of this patent, and all three of which are hereby incorporated by reference. Other welding type power supplies include an input circuit that has an input transformer, and\or derives power from an engine/generator and/or derives power from an energy storage device such as a fuel cell or battery. Welding type power supplies can include additional stages, or use other topologies for each stage (such as a buck preregulator, a combined rectifier-boost preregulator, a chopper instead of or following the inverter, a second inverter following the first inverter, etc.
Inverter based output circuits offer many advantages, but they do have some drawbacks. First, the switches and diodes used in an inverter circuit can fail, particularly when exposed to higher than rated voltages. Clamping voltages is known, but can result in excess losses. Also, commutating switches and diodes can create excess heat. U.S. Pat. No. 6,801,443 returned the entire clamp energy back into the output in one single switching event. US Patent Publication 2014-0263240, owned by the inventor of this invention and incorporated by reference, discloses a way to effectively return commutation energy.
A full bridge output inverter is described in US Patent Publication 2014-0263240. The output inverter is used to invert the dc bus to provide an ac output. An ac weld output typically has a frequency of between about 20-400 Hz. When the output polarity is reversed the current must cross zero. During zero crossing there is a risk that the arc rectifies/extinguishes. A very rapid zero crossing reduces the likelihood of arc rectification.
Prior art U.S. Pat. No. 9,120,172 B2 and application US 20140083987 A1, purport to be able to decrease the amperage rapidly by switching a high impedance path into and out of the weld circuit, thus changing the decay time constant. This passive circuitry dissipates extra heat within the power source—it wastes power and generates unwanted heat.
US Patent Publication 2014-0263240 provides a much better alternative to the prior art that wastes power by helping the arc stay to stay lit during the transition from one polarity to using an assist circuit that applies a high voltage to the output and drives the current up to a certain level. Before inverting the polarity of the main steering IGBT in the AC output, the output inverter switch is left and power to the inverter is removed. The output current then decays slowly (freewheeling through the switch and the transformer). Then the on/off states of the inverter switches are reversed, and power is applied in the new polarity. During this time the output will not have the desired waveshape. Once the IGBTs are switched, the current falls rapidly in an uncontrolled high di/dt manner A high voltage source (in the form of a buck circuit) assisted in arc reversal and driving the current up. Alternatives provide for using additional taps on a transformer for the source of high voltage.
Because US Patent Publication 2014-0263240 teaches that the current decreases naturally (freewheeling through the switch and transformer) toward zero prior to polarity reversal, and that after the current reverses, the assist switch was closed and remained closed, the rate of change of the current (di/dt) was not controlled. Rather, di/dt was set by the assist voltage and components in the circuit. When di/dt rate was not otherwise controlled the current is not at the desired levels while decaying or increasing, and the output cannot follow a desired wave shape. At some predetermined output, around about 100 amps, the assist switch is opened and control reverted back to “normal” operation. Normal control refers to controlling the output magnitude by controlling the input to the inverter.
US Patent Publication 2014-0263240 described a full bridge inverter as the preferred embodiment, and taught it could be implemented as a half bridge inverter. However, there was no control of increasing and decreasing current for both positive and negative output using a half-bridge inverter—the prior art half bridge inverters do not provide four quadrant control. Four quadrant control, as used herein, is a is control of the output in four quadrants—electrode positive with actively driving an increase or decrease of the output current, and electrode negative with actively driving an increase or decrease of the output current.
Four quadrant control can be provided using a full bridge inverter, but that requires four relatively expensive switches, each having to handle the weld current. Additionally, if assist or energy recovery is to be performed, then an additional switch is needed. Thus, the prior art either required more switches (full-bridge) or provide less control of the rate of change of the current (half-bridge). Also, prior art using a high voltage to assist commutation has additional circuitry to create the high voltage. Moreover, because the prior art provided the higher voltage only during output current zero-crossing it was not able to assist the process during transient. Transient, as used herein, is a sudden change in the load or input that cause the control loop to react, such as short circuiting of the arc, a sudden change in arc voltage, a longer arc, a melting electrode, a drop in the input voltage, etc.
FIG. 2 shows a prior art half bridge output inverter welding type output circuit, such as the half bridge version of US Patent Publication 2014-0263240. It includes inverter switches 202 and 203, with anti-parallel diodes 204 and 205. A plurality of rectifying diodes 218 and 219, along with a transformer 221 feed the inverter.
An electrode positive (EP) output is provided when switch 202 is on, switch 203 is off, and the current flows from a center tap on a main transformer windings 222 through an output inductor 214 (as shown by arrow 227), through the electrode output, across the arc to the work output, through switch 202, and back (arrow 228) through rectifying diodes 218 to the main windings 222. Electrode output, as used herein is the output of the machine to which an electrode is typically connected. Work output, as used herein is the output of the machine to which an electrode is typically connected.
An electrode negative (EN) output is provided when switch 202 is off switch 203 is on, and current flows from rectifying diodes 219 through switch 203 (which is on), through the work, across the arc to the electrode, through the output inductor 214 (opposite of arrow 227), and back to the center tap on main transformer windings 222. An electrode positive (EP) output is provided when switch 203 is off switch 202 is on, and current flows from the center tap on main transformer windings 222.
US Patent Publication 2014-0263240 provided for reversal of the current from EP to EN by reducing or removing the power provided to the inverter, and leaving switch 202 on. Inductor 214 causes current for freewheel through inductor 214, switch 202 and winding 222. The current decayed at the rate set by the voltage and the inductances, but is not controlled. When current was reduced to close to zero switch 202 was turned off and 203 turned on, along with assist switch 207.
US Patent Publication 2014-0263240 taught to use assist switches 207 and 208 and an inductor 212 to help increase current magnitude after zero crossing. Assist switch 207 applied the high voltage to assist the current magnitude increase in the EN polarity. The current reversed through the electrode and work (to EN). The EN path included rectifying diodes 219, switch 203 (which was on), the work, the arc, the electrode, output inductor 214 (opposite of arrow 227), and the center tap on main transformer windings 222. The high voltage caused the current magnitude to increase at the rate set by the voltage and inductance, but was not controlled.
FIG. 3 shows the uncontrolled decay of current (di/dt) for an EP to EN reversal in segment 302 and a current increase (di/dt) at a rate set by the voltage source (but otherwise uncontrolled) after zero crossing in segment 301. The EN to EP reversal was done in a similar way. When di/dt is uncontrolled the desired output waveform might not be provided.
Accordingly, a welding type power supply with an inverter output circuit that is clamped in an efficient manner is desirable. Preferably, such welding type power supply would also provide for efficient commutations of devices, be able to handle transients, be able to control di/d during reversals, and/or be able to provide a source of high voltage for assisting commutation and handling of transients.